A point-to-multipoint access optical communication system referred to as “PON system” has been widely used as a method for implementing a public line network that employs optical fibers.
The PON system is constituted by an OLT (Optical Line Terminal) as a station-side apparatus and a plurality of ONUs that serve as a plurality of subscriber terminal apparatuses connected to the OLT via an optical star coupler. Because a number of ONUs can share the OLT and most part of the optical fibers configuring transmission paths, reduction of the operation cost can be expected, and because the optical star coupler, which is a passive component, does not need a power supply and can be easily installed outside, there is another advantage of high reliability. Accordingly, the PON system has been recently actively introduced as a key technology for implementing broadband networks.
For example, in a GE-PON (Gigabit Ethernet®-Passive Optical Network) having a transmission speed of 1.25 Gbit/s, which is standardized compliant with the IEEE802.3ah, a downstream from an OLT to ONUs employs a broadcast communication system using an optical wavelength band of 1.49 micrometers and each of the ONUs retrieves only the data addressed to itself in an allocated time slot. On the other hand, an upstream from each of the ONUs to the OLT uses an optical wavelength band of 1.31 micrometers and employs a time-division multiplex communication system for controlling transmission timing such that data from the ONUs do not collide with each other.
In the upstream communication of the PON system described above, an optical transmission unit in each of the ONUs generates an upstream burst optical data signal according to each transmission timing. In order to generate the burst optical data signal at a high speed, differential driving of a semiconductor laser diode (LD) is effective. For example, Patent Literature 1 proposes a technique related to differential driving of an LD.